1. Field of the Invention
The present invention relates in general to a semiconductor device, and relates in particular to a semiconductor device, which is appropriate for driving an electret condenser microphone.
2. Description of the Prior Art
An electret condenser microphone (ECM) is element, which is used to convert aerial vibrations such as voice to electric signals representing changes in capacitance values. Because its output signal is very weak, an element for amplifying the output signal of the ECM is required to have characteristics of high input impedance, high gain, and low noise.
There are elements that satisfy these requirements, which are the junction field-effect transistor (J-FET) and the metal-oxide semiconductor field-effect transistor (MOSFET). As described in Japanese Laid-Open Patent Publication 58-197885, for example, especially the J-FET element is easily mountable to be integrated in a bipolar integrated circuit.
FIG. 1 shows a cross-section of a p-channel J-FET device. As shown in the diagram, the J-FET device includes a p-type substrate 1; an n-type epitaxial layer 2 deposited on the substrate 1; an n+-type buried layer 3 formed between the substrate 1 and epitaxial layer 2; a p+-type isolation region 4 penetrating from the surface of the epitaxial layer 2 into the substrate 1 and surrounds the buried layer 3 to form an island region 5.
An n+-type top gate region 6 is formed in the surface of the island region 5. A p-type channel region 7 is formed below the top gate region 6. A p+-type source region 8 is formed on one end of the channel region 7, and a p+-type drain region 9 is formed on the other end. Highly concentrated n+-type gate contact regions 10 are formed on the outside of the source region 8 and drain region 9, respectively.
An insulating film 16 is deposited on the top surface of the entire device. A source electrode 11S, drain electrode 11D, and gate electrode 11G are connected to above mentioned regions 8,9,10 respectively through the insulating film 16. The resulting configuration is that of a conventional p-channel J-FET.
According to the p-channel J-FET, a pn junction is formed in the gate region. Hence, the junction can be reverse-biased to control the width of the depletion layer and restrict the drain current.
When integrating other functions in the semiconductor device, a p-type base region 12, an n+-type emitter region 13, and an n+-type collector contact region 14 are formed in another island region 5, which works as an npn bipolar transistor. The npn transistor processes signals received by the J-FET element, acting as an element of overall construction of an integrated network.
However, when the elements above mentioned are used to amplify signals from an ECM, it may be required to provide an extended electrode 15 in the device that has a surface area much larger than that of the device""s electrode pads.
This construction generates a parasitic capacitance C1 between the extended electrode 15 and epitaxial layer 2 sandwiching the insulating film 16 therebetween, and a pn junction capacitance C2 between the epitaxial layer 2 and substrate 1. These capacitances are connected to a substrate-biased ground potential GND. The values of these capacitances can reach as much as several tens of pF, which is a level that cannot be ignored.
FIG. 2 shows a schematic circuit diagram including capacitances C1 and C2. The ECM is connected on one end to a gate (input terminal) of a J-FET 17. The source electrode of the J-FET 17 is grounded. The drain electrode of the J-FET 17 is connected to an output terminal OUT. The output terminal OUT is connected to an integrated network, including an npn transistor or the like that is formed on the same substrate. The capacitances C1 and C2 described above are connected in series between the gate electrode of the J-FET 17 and the ground potential. Accordingly, signals output from the ECM flow to the ground via the capacitances C1 and C2, as illustrated in the diagram by a current i. As a result, the signal level applied to the gate electrode of the J-FET 17 drops, thus the desired output voltage can not be obtained.
Sometimes it is required to add a test pad for measuring the properties of the input transistor during the fabrication process. As shown in FIG. 3, a test pad 18 is formed on the insulating film 16, as with the extended electrode 15 shown in FIG. 1, and connects to the gate electrode 11G of the input J-FET for testing the behavior of the J-FET before shipping. As with the input/output pads of the integrated network, the test pad 18 is usually formed in a rectangular shape with one side measuring 100-300 xcexcm. The p+-type isolation region 4 is formed on the underside of the test pad 18. As a result, a parasitic capacitance C3 is generated by the test pad 18 and the isolation region 4. This capacitance C3 is connected in parallel to the capacitances C1 and C2, as shown in FIG. 2, further increasing leakage in the current flowing to the ground potential GND.
In view of the foregoing, it is an object of the present invention to provide a semiconductor device, which is able to provide a desired output voltage of the ECM without signal loss caused by parasitic capacitances.
To achieve the object of the present invention, there is provided a semiconductor device, comprising: a semiconductor substrate; integrated network elements including an input transistor being integrated on the semiconductor substrate, the input transistor having an input terminal; a first bonding pad connected to the input terminal of the input transistor for testing properties of the input transistor; a second bonding pad connected to one of the integrated network elements for external connection; and a surface area of the first bonding pad being smaller than that of the second bonding pad.
The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate a preferred embodiment of the present invention by way of example.